BASE OP TYPICAL DEVONIAN SYSTEM 335 



determined by William Lonsdale to belong to a marine fauna interme- 

 diate between those of the Silurian and the Carboniferous rocks, and 

 the rocks containing them were therefore constituted a system by Sedg- 

 wick and Murchison in 1838. These fossils mark the rocks which were 

 accepted as the marine equivalents of the Old Red sandstone, to which 

 was applied the name of '• Devonian system ; "* but in this definition of 

 the Devonian system the lower limit, stratigraphically, was not known 

 in Devonshire. Further elaboration of the lower formations and faunas 

 of the Devonian system has resulted from the study of other faunas in 

 other regions. In the Ardennes and Rhenish regions the lowest De- 

 vonian has been called Gedinnian (Dumont, 1848), and in some places 

 rests directly on Cambrian rocks. The Gedinnian is followed above by 

 the Coblenzian (Dumont, 1848). These more western representatives of 

 the European Eodevonian are chiefly shales, sandstones, and grits. On 

 going eastward limestones with marine faunas are met with, and they 

 constitute the Hercynian of the lower Hartz (Kayser, 1870). In Bohemia 

 there is a series of limestones and shales, passing from an unmistakable 

 Devonian horizon (etages G H) downward gradually and conformably 

 into Silurian beds (etage E, Barrande, 1846). In this gradual change 

 in the' petrographic make-up of the formations on going eastward, the 

 question to be determined is the place of the division plane between 

 the two systems, in terms of stratigraphy as well as in terms of fossil 

 faunas. 



On the principle of priority, one fact can not be ignored. The paleon- 

 tological boundary can be settled only by first determining how high up 

 in the strata the Silurian fauna is present. In every step it is the base 

 of the Devonian which is uncertain, and ever}^ determination of a base of 

 the Devonian is subject to revision until it can be shown that it does not 

 transgress the upper boundary of an established top of the Silurian. The 

 Devonian must adjust to that Silurian limit which is already fixed. 



The answer, then, to the first question is clear : 



(«) The upper limit of the standard Silurian system is already estab- 

 lished by definite formations and faunas, and no settlement of any par- 

 ticular case can violate this established precedent. On the other hand, 

 the lower limit of the Devonian in Europe is still under debate, and if 

 lower faunas are discovered which can be shown to be more recent than 

 the Neosilurian faunas, they are to be placed in the Devonian and not 

 added to the Silurian. 



(6) But this further point is established — i. e., in cases where the corre- 

 lation with typical standards is doubtful, reason must be shown to prove 

 that the fauna in question is more recent than standard Silurian faunas 



* Trans. Geol. Soo. (2), vol. v, 724-727 ; Quarterly Journal, vol. viii, p. 3. 



